Various types of incandescent lamps have been proposed with an envelope having a coating thereon having as high a coefficient of transmissivity as possible to light energy in the visible range and as high a coefficient of reflectivity as possible to energy in the infrared range. The envelope is shaped and the filament is shaped and located so that infrared energy is reflected back to the filament to raise its operating temperature thereby requiring less power from the external electrical source to heat the filament to its point of incandescence and thereby resulting in improved efficiency of the lamp.
In general, such lamps have used a variety of coatings, such as, for example, two thin films of a dielectric material which are separated by a film of a highly electrically conductive metal (see U.S. Pat. No. 4,160,929 to Thorington, et al), two layers of a highly electrically conductive metal which is separated by a dielectric (see U.S. Pat. No. 4,409,512 to Walsh); and various types of multi-layer semiconductor coatings, etc.
The envelopes for such lamps are of glass and can have a variety of shapes, for example, spherical (see U.S. Pat. No. 4,160,929) or ellipsoidal (see U.S. Pat. No. 4,375,605). While such lamps are fully operative, it is somewhat difficult from a manufacturers point of view to readily form an envelope having the exact sphericity in the case of a spherical envelope or the proper radii in the case of an ellipsoid or other shape. If the desired shape of the envelope is not exact, or there are irregularities from the desired shape, then the infrared energy which is to be reflected by the coating back onto the filament may miss its target, thereby reducing the lamp's gain in efficiency.
Envelopes for lamps of this type are sometimes formed in two pieces each being pressed in a suitable glass manufacturing operation. However even here, in the case where the interior wall of the envelope where the infrared reflective coating is to be laid down is a continuous curved surface, it is somewhat difficult to obtain the desired accuracy in the shape or degree of smooth surface finish on the envelope interior. Any roughness, depression, projections or other blemish on the interior of the envelope may cause the infrared energy to be reflected in a manner such that it either misses or does not return to the desired part of the filament.